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Exposition d’une nappe d’accompagnement aux contaminants organiques et au changement climatique : Une approche par l’isotopie et la géochimie

Abstract : France draws nearly 40% of its groundwater from alluvial aquifers in direct contact with surface entities. The catchment fields located there exploit a resource in a surface water-groundwater continuum. The impact of anthropogenic activities through contaminants transferred by surface water or the effects of climate change are priority risks that managers are obliged to control in order to guarantee the continuity and quality of their services.This thesis develops an approach to study these risks based on an understanding of the hydrogeological functioning of a typical accompanying groundwater table: the Comps table. Its catchment field continuously exploits the alluvial groundwater of the Rhône and has a simple and adapted configuration allowing the characterisation of determining parameters in the face of these risks. This approach makes it possible to check the consistency of the results between independent tools and to compare their divergences in order to arrive at a synthetic interpretation taking into account their specific characteristics. The knowledge acquired in the course of this work favours intercomparison with similar studies and provides decision-makers with operational tools for resource management.Thus, in the law of the Comps aquifer, the use of geochemical and isotopic tracers and statistical tools reveals that :- the Rhône, and not the Gardon, mainly supplies water from boreholes located as close as possible to the river bank,- A mixture between the waters of the Rhône and those of the karstified limestone massif bordering the Comps tablecloth on its western limit, feeds the boreholes far from the banks,- The transfer time from surface water to the boreholes located near the river bank is characterised by a combination of transfer times ranging from 1 to 120 days, with an average transfer time of 40 days (±10). This configuration evolves significantly during flood periods with faster transfer times that need to be taken into consideration.The exposure of the resource to organic contaminants is therefore subject to this double feeding of the water table, as well as to transfer times spread over several months for contaminants coming from surface water.The monitoring of the different water masses shows an almost continuous exposure of the water table to 11 of the 31 contaminants analysed via its two sources of supply. These are compounds from urban, industrial, agricultural and hospital activities at concentrations of the order of ng/L: caffeine, carbamazepine, sulfamethoxazole, sotalol, salicylic acid, metformin, acetaminophen, atrazine, DEA, simazine and PFOA.During their transfer into the water table, the fate of these contaminants highlights the existence of natural protection mechanisms: reactive degradation-sorption and dilution processes. However, the effectiveness of these mechanisms varies according to the characteristics of the contaminants and the hydrogeological conditions. The evaluation of these processes makes it possible to define a permanent coordination of the configuration of the boreholes in the catchment field in order to meet the needs of the community by optimising the exploitation capacities of the resource associated with a maximum reduction for these eleven contaminants.Furthermore, the contribution of Alpine waters in the Rhône was evaluated at 70 ± 6% in the summer period (mid-July to early September for the years studied). This contribution, which is potentially essential to the sustainability of the river, is essential for the supply of the Comps catchment area.
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Submitted on : Tuesday, March 23, 2021 - 6:11:08 PM
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Julien Jean-Baptiste. Exposition d’une nappe d’accompagnement aux contaminants organiques et au changement climatique : Une approche par l’isotopie et la géochimie. Hydrologie. Université de Nîmes, 2020. Français. ⟨NNT : 2020NIME0003⟩. ⟨tel-03178442⟩

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